Use of tones is widespread in telephony. They are used in setting up a telephone call and to indicate the progress of the call. More recently, tones have been employed during calls to effect advanced features and/or functions. One example is the use of dual-tone multifrequency (DTMF) signals to control the addition of one or more individuals during a conference call.
A serious problem with prior tone detectors is that they falsely detect speech, music or data as tones used for other purposes. That is, the speech, music or data emulates either the individual tones or the DTMF signals. Such false detection of tones during a telephone call causes a so-called "talk-off" condition, i.e., a disruption of the call, resulting in a failure of the communications circuit. Another problem with prior arrangements is that valid tones or DTMF signals are falsely rejected in the presence of channel impairments, e.g., low signal-to-noise ratio.
A recent attempt at overcoming the talk-off problem and the falsely rejecting valid tones or DTMF signals problem employs a plurality of discrete prolate spheroidal sequence (DPSS) tapers, i.e., data windows, to slice a received signal into time segments. Then, Discrete Fourier Transforms (DFTs) of the tapered segments are taken to form estimates of the received signal energy in the frequency domain. The frequency domain energy estimates are then used to identify the received tone or DTMF signals.
Although this prior arrangement operates satisfactorily in certain applications, it still presents a significant talk-off problem when designed to operate satisfactorily in detecting valid tones or DTMF signals in the presence of channel impairments. This is primarily caused by only evaluating frequency domain energy and by the required use of a relatively wide-band filter in the frequency domain to accommodate the use of the plurality of DPSS tapers. Use of such a wide-band filter allows speech, music or data to easily emulate the individual tones and/or DTMF signals, thereby causing talk-off.